The present invention relates to a rear projection screen suitable for use as, for example, a screen for video projectors and, more particularly, to a rear projection screen having a greater angular field of vision on the viewing side and an increased brightness.
Rear projection screens have been widely available for video projectors, microfilm readers and computer display systems, and various studies and attempts have been made for improving the light transmitting characteristics of the rear projection screen to attain a greater angular field of vision. One of the measures for achieving such aim is to use, solely or in combination with a lens or a diffusion plate, a lenticulated surface having a multiplicity of minute cylindrical lenses (lenticules) arranged contiguously.
The screen having the lenticulated surface is effective in diffusing the light impinging thereon. More specifically, a lenticulated surface having a multiplicity of minute vertically extending cylindrical lenses arranged contiguously on a vertical plane diffuses the light laterally, while the lenticulated surface having a multiplicity of minute horizontally extending cylindrical lenses arranged contiguously on a vertical plane diffuses the light longitudinally. When this lenticulated surface is used as a screen, the maximum diffusion angle is varied largely depending on whether the lenticulated lens faces the incident light, i.e. the light source, or the viewer. Namely, as is well known to those skilled in the art, it is possible to obtain a greater diffusion angle when the surface faces the light source than when the same faces the viewer.
In general, however, there is a practical limit in the angular field of vision if each lenticular lens unit has a simple circular cross-section, even when the lenticulated surface is directed towards the viewer. This is attributable to the fact that the angular field of vision is restricted in order to prevent a large loss of light in the portions of large incidence angle in accordance with the condition of critical angle and Fresnel's formula, when the light impinges from the projecting side as a parallel light beam.
More specifically, assuming here that a parallel light beam L impinges as illustrated in FIG. 19, total reflection takes place when the following condition is met: EQU n.multidot.sin .theta.=sin .phi.=1
where n represents the refractive index of the medium.
Thus, the light impinging at an angle .theta. greater than this incident angle is not transmitted to the viewer's side.
The reflection factor r at the interface in the Fresnel's formula is defined as follows. ##EQU1##
This reflection factor r takes a large value when the incidence angle is near the critical angle.
As understood from the foregoing description, at the lenticulated surface consisting of lenticules having simple circular cross-section, the quantity of light is reduced almost to zero in the region of about 30.degree. from the center as shown in FIG. 20, and the image cannot be observed in the region of angular field of vision exceeding the above-mentioned angle.
It has been proposed to make the lenticulated surface have a parabolic cross-sectional shape. A too high quality cross-sectional shape of the lenticulated surface, however, takes much time and labor and, hence, increases the cost in the manufacture of the mold. It is difficult to produce the screen at a high reproducibility of the screen shape, even at the cost of much labor and time employing an expensive mold.